1. Field of the Invention
The present invention pertains to the field of methods and apparatus involving electronic air filtration devices. More specifically, these devices apply an electric field to polarize a filtration medium, in order to increase the filtration efficiency of the medium.
2. Statement of the Problem
Four of the top ten health problems in the United States are related to respiratory conditions that can often be alleviated by the use of an air filtration device. These problems, in order of their problem ranking, include: #1 sinusitis, #5 allergies, #7 bronchitis, and #8, asthma. Nevertheless, less than 2% of the estimated 94 million households in America currently own an air purifier. Conventional air purifiers are characterized by a variety of performance deficiencies. These filters fail to satisfy volumetric demands, are noisy and expensive to operate, or fail to provide an adequate particle removal efficiency. These performance problems have created a clear market need for the introduction of a superior air purifier at a reasonable cost.
Air filtration system designers must balance the need for high filtration efficiency against the energy requirements of pushing air through an increased resistance to air flow that is associated with the use of higher efficiency filtration media. A significant problem with conventional electronic air filtration systems is that their airflow throughput and overall efficiency often decreases as the filtration medium collects pollutants, such as particles, liquids (e.g., condensed atmospheric water), and microorganisms. The level of decreased efficiency can be significant, which results in a dramatically lower overall air cleaning benefit.
More energy is required to push air through a filter having a higher filtration efficiency derived from smaller openings. This increase in energy consumption derives from the fact that the volume of air that is moved through the filter decreases proportionally with resistance (i.e., from the smaller openings) against the volume of air flowing through the filter. Fans that are capable of moving a large volume of air against a high filter resistance are significantly more expensive, much noisier, and require more energy to operate. Purely mechanical filters that do not utilize induced electrostatic forces to enhance their efficiency are particularly burdened by air resistance problems because the filtration efficiency of these filters cannot be increased without also increasing the number of fibers in the filtration medium. The resistance to air flow increases with the number of fibers in the filter. Resistance also increases with a decrease in the average pore size openings of non-fibrous filtration media.
In recent years, very few improvements have been made in either the technology of electrostatic air filtration or the design of existing air purifiers. Conventional air filtration systems utilize two basic methods for air purification. A first method utilizes mechanical filters that consist of a flat or pleated mat of fibers contained in a supporting frame. A second category of air purifiers uses electronic or electrostatic technology to enhance the performance of the filtration medium.
Electrical air filters obtain a higher filtration efficiency from a given mechanical filter because electricity is used to induce a polarization state in the fibers of the filtration medium. The applied electric field also induces a polarization state in at least some of the particles within the airstream to be filtered. The electrostatic forces in the particles and the filter medium attract one another to bind the particles to the medium. These forces of electrostatic attraction can increase the filtration efficiency of a given filtration medium by several fold.
By way of example, a mechanical filter generally consists of a flat or pleated mat of fibers. The filter is contained in a supportive frame. The filter removes particles from air passing through it by collecting the particles as the particles contact individual fibers, or the particles are too large to pass between a plurality of fibers. The percentage of particles that are trapped determines the overall filtration efficiency, e.g., 4%, 20%, 50%, or 85%. A typical furnace filter that is used in household furnace applications is one having a thickness of about one inch. This type of filter offers an extremely low resistance to air flow, and has a very low efficiency on the order of 4-9%. This filtration efficiency can be increased to about 40% by polarizing the filter between two conductive electrodes, one of which is charged to about 14-15 kV and placed in contact with the filter.
Conventional electronic air filtration systems draw in air through a front section that imparts a positive charge to particles in the incoming air. The air and charged particles are subsequently passed between a series of plates that sequentially alternate between parts having a positive charge and grounded plates. The positive particles are repelled from the positive plates, but collect on the grounded plates. These systems typically have a very low resistance to air flow because of their open configuration.
U.S. Pat. No. 3,915,672 (1975) discloses an electrostatic precipitator having parallel grounded plate electrode dust collectors. High voltage corona wires are located between the plate electrodes. The corona wires charge the dust particles, which are then drawn to the plate electrodes. The corona wires are pulsed to prevent corona back-charging that would, otherwise, occur due to the high resistivity of the dust accumulation on the plate electrodes.
U.S. Pat. No. 5,055,118 (1991) to Negoshi et al discloses an electrostatic dust collector. A first positive ionization electrode positively ionizes dust in the incoming air. The ionized dust and air pass into a chamber having a pair of uninsulated electrodes, which are maintained at a high voltage. The electrodes are separated by an insulation layer. Columb's Law causes the dust to collect on the ground electrode where the positive charge on the dust is neutralized. The dust only collects on the grounded electrode because special gaps in the laminate prevent dust build-up on other components. Cleaning of the negative electrodes is necessary to maintain airflow.
A manuscript entitled "Electric Air Filtration: Theory, Laboratory Studies, Hardware Development, and Field Evaluations" by Lawrence Livermore National Laboratory (1983) reports various experiments in the field of electrostatic air filtration technology. The report states that an electrically enhanced filter is an ideal candidate for removing sub-micron airborne particles because an electrified filter has a much higher filtration efficiency than does a conventional non-electrified fibrous filter. The electrically enhanced filter also has a significantly lower pressure drop at the same level of particle loading, and a greatly extended useful life.
The above-identified Lawrence Livermore Laboratory report disclosed a preferred filtration system having an uninsulated electrode that was placed in front of a fibrous filter. A grounded, uninsulated electrode was placed downstream of the fibrous filter. The upstream electrode was charged to create an electric field across the fibrous filter. The applied field induced a polarization state along the respective lengths of individual fibers of the filtration medium. Thus, the fibers collected either positive or negative particles all along their lengths on both sides of the fibers because a positively or negatively charged portion of a fiber served to attract an oppositely charged portion of a particle. The filtration efficiency and longevity of the electrically enhanced filtration medium were excellent. The filtration efficiency was shown to be dependent upon the strength of the electric field between the electrodes. The strength of the electric field increases with high electrode voltages for a given distance between the electrode.
The upper limits of a field strength that may exist between two uninsulated electrodes which are retained a fixed distance apart constitutes a limiting factor of the Lawrence Livermore filtration system design. Voltage tends to arc between the electrodes when the voltage or potential between the electrodes exceeds a threshold level. The arcing can burn holes completely through the filtration medium. The arcing also constitutes a temporary short circuit between the electrodes and, consequently, substantially eliminates the benefits of the field that formerly existed between the two electrodes. The exact value of the arcing threshold level varies with the degree of contamination on the filter medium. This contamination includes, among other things, dust particles and water precipitate from the air. Thus, the system might work with an electrically enhanced efficiency when the relative humidity was very low, but would fail when the relative humidity value was very high. The Lawrence Livermore test data reports arcing at a 12 kV potential between electrodes spaced about one-half inch apart across a fibrous filter.
The Livermore study attempted to overcome the arcing problem through the use of insulated electrodes. This attempt failed because trapped charges eventually neutralized the effect of the applied field. Charged particles tended to collect or migrate onto the filter surfaces proximal to an electrode having an opposite charge with respect to that of the particles. Thus, a corresponding trapped charge grew on the filter surfaces proximal to the insulated electrodes. The trapped charge had the effect of reducing the applied field that was able to reach the filter medium. This deleterious effect is known in the electronics industry as `screening` of the applied field because the field coming from its electrode origin interacts with the trapped charge in such a way as to reduce the magnitude of the applied field that is able to reach positions located downstream of the trapped charge.
The performance of the Livermore filtration system using insulated electrodes deteriorated drastically as opposite charges built up and substantially neutralized the applied electric field (see the Livermore report on page 103). Persistent arcing between the electrodes prevented the model from becoming commercially feasible. Thus, the insulated electrodes prevented the arcing problem, but caused a decline in the filtration efficiency as collected charges neutralized the applied field. The Lawrence Livermore report, accordingly, indicated that uninsulated electrodes having high resistivity might provide a satisfactory solution to the problem.
U.S. Pat. No. 5,330,559 (1994) teaches the use of a non-deliquescent foam (one that does not attract water) that is sandwiched between an uninsulated high resistivity electrode and an uninsulated ground support frame. Incoming air is exposed to an ionizer that serves to charge particles in the air. The high resistivity electrode fails to prevent shorting or arcing between the high resistivity electrode and the ground plate. This design fails to prevent shorting or arcing between the electrode and the ground (or between the two electrodes). Thus, the filtration system utilized a non-deliquescent foam in an effort to overcome the arcing problem and, specifically, arcing problems that derive from high levels of relative humidity.
There remains a true need for method and apparatus that overcome the problem of arcing between the electrodes while permitting higher filtration efficiencies derived from insulated electrodes. The present inability to apply higher field values constitutes a limiting factor in the development of further efficiency enhancements in the field of electronically enhanced air filtration technology.